1. Field of the Invention
The present invention relates to a method and apparatus for starting a plasma arc treatment system, and more particularly, to a plasma torch system that operates in a non-transferred arc (NTA) mode and a transferred arc (TA) mode without changing any mechanical element of the plasma device.
2. Description of the Prior Art
Plasma reactors have been the subject of numerous research and development projects, and often of patents, over the last several decades. By definition, such reactors make use of a plasma gas, forming a heat-generating arc column between two or more electrodes to heat the material to be processed to high temperature, and thus allow desired reactions to occur that would not be otherwise obtainable or economical. The plasma gas forming the arc column consists of a mixture of energetic and/or disassociated molecules, positively charged ions and free electrons obtained from the gas that is subjected to partial ionization by means of an electric arc (usually D.C.) formed between an anode and a cathode. Plasma arc treatment systems are used in applications such as metal melting, powder production, and hazardous waste treatment.
In practice, the plasma gas may often be used as a reactant. Thus, by way of example, oxygen or air may be used for carrying out oxidation. Carbon monoxide or hydrogen may be used for carrying out reduction. Chlorine may be used for carrying out chlorination and nitrogen for nitration.
In the plasma arc treatment chambers, a plasma device transfers electrical energy through a stream of gas so hot that the gas becomes an electrical conductor. The commonly owned U.S. Pat. No. 4,912,296, for example, discloses an advantageous construction for a plasma torch processing system. U.S. Pat. No. 4,770,109 and U.S. Pat. No. 5,136,137, both by the inventor of the aforementioned invention and also commonly owned, disclose and claim reactors for the treatment and melting of all types of materials, particularly hazardous waste, for which the present invention is particularly useful. Both patents are hereby incorporated by reference for all purposes.
Generally speaking, there are two types of plasma devices: non-transferred and transferred. In non-transferred arc devices, both electrodes are contained entirely within the device, for example, between two coaxial rings such that an electrical arc forms in the annular space between the coaxial rings. A gas is passed through the annular area and emitted from an end of the torch.
In transferred arc devices, one electrode is contained in the device and the other electrode is exterior and spaced apart from the device. The other electrode also is usually at the surface of the material to be treated and/or heated. In many circumstances, transferred arc devices are more efficient than non-transferred arc devices.
In the treatment of hazardous waste, equipment suitable for treating waste as described in the aforementioned patents includes a generally cylindrical tub open at the top, rotating about a vertical axis within a sealed chamber, a system for charging material into the tub, a movable plasma arc device mounted above the tub (referred to subsequently as the centrifuge) and electrical connections from an arc power supply to the plasma device and to the conductive base of the centrifuge.
A very important element of the waste treatment process is to melt the inorganic (usually oxide) components of the feed into a slag while evaporating water, organics and most salts. Such a slag is electrically conductive at high temperatures and non-conductive at low. Since the conductive bottom of the centrifuge may be covered by non-conductive slag if the process operation is interrupted, a way to transform the non-conductive layer to the conducting state is needed. The present invention is particularly effective for this purpose.
When the plasma arc treatment system is shut down, an amount of the slag, i.e., a slag skull, is left in the drum to form the slag for a subsequent use. During the down time, the slag cools and may solidify. As the slag temperature drops, the electrical conductivity of the slag also decreases. A problem that occurs when starting the plasma arc treatment system is that the electrical conductivity of the slag may have dropped to a level that will not sustain an arc between the torch and the grounding network. In order to start the treatment system and sustain an arc, the slag must be heated to increase the conductivity.
A conventional method of heating the slag is with an oxyacetylene torch or a non-transferred plasma arc device. In such a device, the plasma arc column permanently extends between the two "built-in" electrodes of the device, even if this column may be blown out of the same by the injected plasma gas, and thereby form an elongated loop.
Plasma arc treatment systems have been developed that include a plasma device that can operate in either a non-transferred arc mode or a transferred arc mode. In such systems, parts must be changed in order to switch between the two modes. This is time consuming and additionally can allow the slag to cool, thus lowering its conductivity. These systems also require an operator to switch between the modes in some type of physical and mechanical fashion. Therefore, these systems are subject to operator error and set-up error. Additionally, they often operate in a preset manner, i.e., the time for switching is predetermined, and thus if this predisposed time is wrong, then valuable time can be wasted in switching between modes at an inappropriate time, i.e., too early, and thus the slag is not yet in a conductive state, or, alternatively, too late and the more efficient and more desirable transferred arc mode is not entered soon enough.
An even bigger problem are systems that do not operate in both modes. In one such system, the non-conductive materials, such as glass, are usually chipped away in order to find a ground path for the transferred arc. This chipping procedure is time consuming and damages refractory systems that line the internal walls of the plasma arc treatment system. Graphite or other electrically conductive metallic rods have also been used with success but have a short operational life in oxidizing environments.